The present invention relates in general to a cleaning machine for cleaning the surfaces of semiconductor wafers. More particularly, the present invention relates to an adjustable brush assembly for cleaning semiconductor wafers.
Semiconductor wafers are typically cleaned in a cleaning machine which includes one or more brush stations each having a pair of rotary brushes for cleaning the upper and lower surfaces of the wafer. When the wafer is positioned between the brushes, the rotary brushes apply pressure across the wafer surfaces to clean the wafer. A pair of stop assemblies engage the forward edge of the wafer to retain the wafer between the brushes and spin the wafer so that the entire surface area may be cleaned by the brushes. One or more liquid agents are sprayed onto the wafer or the brushes to facilitate cleaning of the wafer. After a set period of time, the stop assemblies are pivoted away from the wafer and the wafer is allowed to pass through the brush station into either another brush station or a final spin station where the wafer is rotated at speeds of approximately 2000 rpm to dry the cleaned wafer.
The cleaning pressure applied to the wafer by the upper and lower brushes is dependent upon the thickness of the wafer and the spacing between the brushes. Preferably, the surfaces of the upper and lower brushes are substantially parallel so that a uniform pressure may be applied to clean the wafer surface. In the brush stations of the available cleaning machines, the lower brush is typically retained in a fixed position. The upper brush is secured to a rigid brush carriage assembly having a transverse bar pivotally mounted to the frame of the machine. The orientation of the brush carriage is manually calibrated so that the upper brush is substantially parallel to the lower brush. Micrometers positioned at opposite sides of the brush carriage are used to manually adjust the height of the brush carriage to obtain the desired spacing between the brushes. This spacing determines the pressure that will be applied to the wafer.
Manually calibrating the position of the brush carriage each time wafers having different thicknesses are cleaned or the brushes are changed is a tedious, time consuming, inefficient and labor intensive process. The micrometers often become jarred during operation of the cleaning machine, moving the brush carriage out of alignment and requiring readjustment of the brush assembly to ensure that the desired cleaning pressure is uniformly applied across the wafer surfaces. Since the brush carriage appears to be a rigid assembly, the operator often elects to use only one micrometer to calibrate the position of the brush carriage. As only one micrometer is then used to support the brush carriage during the cleaning operation, the carriage may deform and more frequent calibration of the brush carriage may be required. The pressure applied to the wafer surfaces changes as the brushes wear, requiring readjustment of the spacing between the brushes. If the brushes do not wear evenly, both the orientation and height of the upper brush must be recalibrated to maintain a uniform pressure across the wafer surfaces. Since the thickness and surface topology of semiconductors wafers varies, more pressure may be applied to some wafers than others. Moreover, if the wafer thickness is not uniform, the brushes will not apply an even pressure across the wafer surfaces. Even if the cleaning machine is closely monitored, many wafers may leave the brush station in an unsatisfactory condition.
The present invention provides an adjustable brush assembly which includes an automated pressure adjustment assembly, such as a support assembly, coupled to a control system for automatically adjusting the pressure applied to the wafers by the brushes. In one embodiment, the control system may be configured with a predetermined height schedule which accommodates predicted changes in spacing due to brush wear. In another embodiment of the invention, the control system has a closed-loop configuration which measures the pressure applied to the wafers during the cleaning cycle and automatically adjusts the brush assembly to ensure that a uniform pressure is applied to the wafers throughout the operating cycle of the cleaning machine. Automatically adjusting the height of the upper brush substantially minimizes the time and labor required to prepare the brush station for operation. Moreover, by adjusting the brush assembly during the cleaning cycle, the cleaning pressure applied to the wafers may be more precisely controlled.